As mentioned by @aszlig, it's more elegant and sane to use coreutils'
`install`. This commit also changes `platforms` to `unix` since this
should also compile on Darwin for example.
We have a mirror:// URL for sourceforge stuff so we can still retreive
the source tarball even if downloads.sourceforge.net is down.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>